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J. Schallhorn, M. Tang, Y. Li, D. Huang; Corneal Refractive Index Changes in Keratoconus. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4345. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To study the effect of keratoconus on the corneal index using optical coherence tomography (OCT) and Orbscan.
12 eyes of 8 patients with keratoconus and 25 eyes of 16 normal patients were imaged using the Orbscan slit-scanning device and the Visante OCT device. The central corneal thickness was measured with both instruments and the ratio was calculated. According to optical principles, the apparent corneal thickness measured by OCT scales in direct proportion to the group index and the apparent corneal thickness measured by slit-scanning scales inversely with the refractive index.
The ratio of the OCT/Orbscan central pachymetry in normal eyes is 0.986 ±0.030 and the ratio in keratoconic eyes is 1.011±0.034. The two are significantly different from each other (p=0.042). Assuming the group and refractive indices changes equally, keratoconus increase index by 1.3% on the average.
Keratoconus appears to increase the corneal index. One possible mechanism is increased corneal stress and strain due to the ectasia. This might lead to more tightly packed collagen lamellae and higher refractive and group indices.
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